An IPv6 address is composed of 8 hextets with 65536 (2^16) possible values for each hextet, creating a total address space of 65536^8, which is the same as 2^128.

A hipku has 16 slots for one-syllable words with 256 possible words for each slot, creating an address space of 256^16, which is the same as 2^128.

Put another away, by having 256 possible choices for each word and choosing 16 words, there are the same number of possible combinations as there are IPv6 addresseses, so we can uniquely identify each IPv6 address with one 16 word combination. Hipku goes one extra step and stores those 16 words in a format that reads as a haiku.